A standoff between police and anti-war demonstrators near the NATO summit at McCormick Place left several protesters bloodied and led to dozens of arrests, as delegates gathered for the first of two days of meetings.

The confrontation, which flared up late this afternoon near the corner of Michigan Avenue and Cermak Road, lasted for almost two hours, as officers in full riot gear went nose to nose with demonstrators, with police determined to keep the crowd away from the convention center where NATO delegates were meeting.

It was not immediately clear how many people were injured in the tussle, but by 7 p.m., seven protesters had been treated at Mercy Hospital and Medical Center, hospital spokeswoman Connie Murphy said.

By 9 p.m. there had been at least 45 arrests, authorities said. There were four police officers with minor injuries, authorities said.

Although officials were able to clear the McCormick Place area, as darkness fell, rogue groups of protesters flared up across the Loop, keeping police scrambling.

Police seemed intent on keeping the ever-morphing packs of demonstrators south of the Chicago River. Police buses lined up along the Michigan Avenue Bridge, as large numbers of officers spread out through the area.

At Van Buren and Wabash streets, police attempted to block a group of more than 100 protesters from heading north on Wabash. The protesters ran through their barricade, some getting into scuffles with police, who detained several protesters.

The rest of the group kept moving.

As one group of protesters marched south on Michigan, several hundred officers rushed to guard the doors at the Art Institute, where First Lady Michelle Obama was scheduled to host a dinner earlier this evening.

Most of the protesters passed the museum and continued walking south, but some stayed behind, saying they wanted to "meet the Obamas."

The evening activity followed a pitched battle between an overwhelming number of police and several hundred protesters outside McCormick Place. The skirmish broke out shortly after 4 p.m., at the end of a day-long protest that started in Grant Park and ended at a makeshift stage at Michigan and Cermak.

"NATO'S East! NATO's East!" the protesters chanted as they moved closer to the makeshift stage where rally leaders had spoken.

A rally organizer told protesters that they had a street permit, but it had expired. He urged the crowd to leave.

"The police are going to move us off the street. We've been telling you this would happen," the organizer told to the crowd.

Activist Andy Thayer used a bullhorn to implore the crowd to move.

"We went to a lot of trouble to make sure this march was a safe and peaceful march," he shouted.

Police made announcements in both Spanish and English warning they might use water or sound or other anti-riot measures to disperse the crowd.

"You are hereby ordered to disperse," police said. "If you fail to comply you may be arrested or be subjected to other police action."

Some protesters chanted "this is a peaceful protest," but others threw plastic bottles at police.

The situation devolved quickly.

Officers lined up four deep in a show of strength, hoping to keep the demonstrators at bay. As the crowd surged forward, some officers swung their batons.

Police Superintendent Garry McCarthy, dressed in his white uniform and wearing sunglasses, walked behind the line of officers. Along the front line of the conflict, police and protesters yelled at each other, sometimes nose to nose. On several occasions, police, holding their batons horizontally, shoved the crowd back.

Several protesters who fell to the ground or resisted were plucked from the crowd and arrested.

Police created a detention area south of the standoff, at Michigan and 21st Street. Officials made a makeshift corral using CTA buses and police vehicles. Detainees in handcuffs were being brought in by police and videotaped as they were processed and loaded into vans.

By early evening, several dozen arrestees had been processed there.

Police are protecting the area from protesters who are not detained but are coming over to yell at the officers. Some are also shouting encouragement to protesters being loaded into police vans.

"We love you," one screamed as one woman was loaded.

"What's your name?" another shouted.

One protester, partially slumped over, appeared to be dehydrated. An officer gave him water.

Two people were detained by the Secret Service for attempting to get over protective fencing near 25th Street and Michigan Avenue, according to George Ogilvie, a Secret Service spokesman.

President Barack Obama left the NATO summit at McCormick Place at 5:30 p.m. and headed north toward his hotel, his motorcade never passing within sight of the action outside the convention center. He was back at the hotel awaiting a formal dinner tonight at Soldier Field.

While police and protesters were clashing, barricades created a no-man's land on the streets leading to nearby McCormick Place, which kept the action a safe distance from NATO delegates.

The confrontation came at the end of what had been a peaceful demonstration that began earlier in the morning at the Petrillo Music Shell and ended several blocks from McCormick Place.

During the afternoon, a group of veterans had taken the stage holding an American flag.

Some in the crowd chanted "Burn that flag! Burn that flag!" The veterans then folded the flag and handed it to Mary Kirkland, who said her son killed himself after two tours in Iraq.

Kirkland said the Department of Defense said her son, Derrick, was "killed in action."

"They start with lies and continue with the lies," she told the crowd. "I'm proud to be here."

Demonstrators are hoping their anti-war message will penetrate the walls of the nearby convention center, where NATO dignitaries have begun their summit.

To do so, the veterans took the stage in small groups, and then threw their war medals to the ground. Each gave a short speech about their decision to "return" the medals.

"If I want to continue to live with integrity, I must get rid of this," said Erica Sloan from Ohio, who said she served in the U.S. Air Force.

Another soldier, who identified himself only as Jerry from New York, tossed his medals onto Cermak Road.

"I choose human life over imperialism and materialism," he said.

In the crowd, protesters chanted "peace, peace, peace," and made the peace sign with their hands.

Earlier in the day, many protesters made it clear their beef was with NATO's policies, not police.

"I think we're seeing a lot of different concerns," said Lillian Moats, 65, of Downers Grove earlier as she looked out over a diverse sea of people including self-described anarchists dressed in black and people made up as clowns.

"I think that there's just a great resonance with the Occupy movement, because if we weren't spending such outrageous amounts on war, we have money for human needs," Moats said. "It seems like our country's priorities are upside down."

Moats said she wonders whether the wider world will be able to grasp a central theme to today's protest given the multitude of issues demonstrators are protesting.

"I certainly hope it will be so, but I also know we live in a world where sound bites are important and so much of the message gets lost," she said. "It's a complicated world and there is more to be written about this than one sentence. I do think there is a very basic correlation between military spending and the lack of funding for human needs, so I'm hopeful that will be the common theme that ties all the other messages together."

At Grant Park, where the day's events began, all of those in attendance were protesting particular injustices. Police estimated the crowd at about 2,000 people. Others put the number at roughly twice that.

Saba Mather, 24, of Columbus, Ohio, said she believes today's rally will make a difference because NATO leaders inevitably will hear protesters' speeches.

"Our message will get to NATO because it will get to the people of the world," she said.

As the crowd at Grant Park heard speakers denounced the war in Afghanistan, President Obama and Afghanistan President Hamid Karzai met reporters at McCormick Place, offering a slightly different take on the war.

"We have ended our combat role  the Afghan War as we know it has ended," Obama said.

"Afghanistan, as you rightly put it, is looking forward to an end to this war," Karzai said. He said he looked forward to the day when "Afghanistan is no longer a burden on the shoulders of the United States and other allies."

A short time later, the summit opened at McCormick Place with a moment of silence in honor of those who have died in service to NATO. Obama bowed his head as a bugler played "Taps."

At the Grant Park rally, McCarthy told reporters early in the day that an overwhelming majority of the protesters have been compliant and have listened to police direction.

The police chief gave his officers high marks for the week, including their response during a handful of flare ups with marchers in the Loop Saturday night.

"I feel very good about the way the officers have handled this incredible amount of stress," he said.

As he spoke, a number of protesters crowded behind him and tried to talk over him.

"Bottom line is what you're seeing is exactly what we've been telling you you would see...officers facilitating peaceful protest, protecting people, providing public safety, while at the same time being intolerant of crimes being committed," he said.

A short time later, around 1 p.m., officers arrested seven men and women near the intersection of Michigan Avenue and Harrison Street.

A police sergeant said the group was carrying about 20 rocks, cans of spray paint, pry bars, urine-filled bottles and other "dangerous weapons." The group was detained in the shade while they were taken one-by-one to be cuffed, searched and placed in a Cook County sheriff's police bus.

Several of the arrested said they did not know why they were being charged and that they were in the park to protest the NATO summit.

On the Petrillo stage, Zoe Sigman of Occupy Chicago addressed the crowd by saying she lived in the Bridgeport apartment raided by Chicago police on Wednesday night. Three people were charged with terrorism-related offenses after police said they were found with Molotov bomb-making materials in the raid.

Sigman said the raid was "illegal" and described the nine people originally arrested as "all my friends."

"They're people like you and me. People here to protest," she said.

As she spoke, the crowd cheered repeatedly, but booed whenever she mentioned the Chicago Police Department. She said the police had no warrant and that other raids have occurred this week. But a warrant for the Bridgeport raid was filed May 16, the day of the raid, according to court documents.

"I'm scared. I don't have a safe place to stay at night because there is no safe place to stay at night," she said. "They violated our privacy, they stole our security...I choose to fight back!"

The Rev. Jesse Jackson appeared with protesters and said the demonstrations must remain non-violent if they are to be successful.

"We learned from Dr. King in Birmingham," he said. "We march in a disciplined, non-violent way. We cannot afford to have our message hijacked by acts of provocation."

There were moments of levity among the protesters as well, as the 90-degree heat sent some scrambling for shade, and others, well One man spent a half hour dancing like a robot, even though no music was playing.

Hundreds more recorded the speeches, recorded themselves commenting on the speeches, or interviewed others just feet from the speakers.

Although most of the message was anti-war, others brought their own agendas, from increasing green jobs to gay rights to immigration rights and fighting against bank foreclosures.

Signs behind the stage included messages for "peace, jobs, equality" to freeing Bradley Manning, the private who leaked military records to WikiLeaks.

Military veterans at an anti-NATO rally threw away their medals on Sunday which they said symbolized their rejection of the U.S.-led wars in Iraq and Afghanistan. Photos from the NATO protests: Monday, May 21 | Sunday, May 20 | Saturday, May 19 | Friday, May 18 | Thursday, May 17 | Chicago...

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